The present invention relates to a linear accelerator.
Linear accelerators, particularly of the standing wave design, are known as a source of an energetic electron beam. A common use is the medical treatment of cancers, lesions etc. In such applications the electron beam either emerges through a thin penetrable window and is applied directly to the patient, or is used to strike an X-ray target to produce suitable photon radiation.
It is often necessary to vary the incident energy of the electron beam fore either type of treatment. This is particularly the case in medical applications where a particular energy may be called for by the treatment profile. Linear standing wave accelerators comprise a series of accelerating cavities which are coupled by way of coupling cavities which communicate with an adjacent pair of accelerating cavities. According to U.S. Pat. No. 4,382,208, the energy of the electron beam is varied by adjusting the extent of coupling between adjacent accelerating cavities. This is normally achieved by varying the geometrical shape of the coupling cavity.
This variation of the geometrical shape is typically by use of sliding elements which can be inserted into the coupling cavity in one or more positions, thereby changing the internal shape. There are a number of serious difficulties with this approach. Often more than one such element has to be moved in order to preserve the phase shift between cavities at a precisely defined value. The movement of the elements is not usually identical, so they have to be moved independently, yet be positioned to very great accuracy in order that the desired phase relationship is maintained. Accuracies of xc2x10.2 mm are usually required. This demands a complex and high-precision positioning system which is difficult to engineer in practice. In those schemes which have less than two moving parts (such as that proposed in U.S. Pat. No. 4,286,192), the device fails to maintain a constant phase between input and output, making such a device unable to vary RF fields continuously, and are thus reduced to the functionality of a simple switch. They are in fact often referred to as an energy switch.
Many of these schemes also propose sliding contacts which must carry large amplitude RF currents. Such contacts are prone to failure by weld induced seizure, and the sliding surfaces are detrimental to the quality of an ultra high vacuum system. Issues of this nature are key to making a device which can operate reliably over a long lifetime.
The nature of previous proposed solutions can be summarised as cavity coupling devices with one input and one output hole, the whole assembly acting electrically like a transformer. To achieve variable coupling values the shape of the cavity has had to be changed in some way, by means of devices such as bellows, chokes and plungers. However the prior art does not offer any device which can vary the magnitude of the coupling continuously over a wide range by means of a single axis control, whilst simultaneously maintaining the phase at a constant value.
The present state of the art is that such designs are accepted as providing a useful way of switching between two predetermined energies. However, it is very difficult to obtain a reliable variable energy accelerator using such designs. A good summary of the prior art can be found in U.S. Pat. No. 4,746,839.
Our earlier application No. PCT/GB99/00187 describes a novel form of linear accelerator in which there are a plurality of resonant cavities located along a particle beam axis, at least one pair of resonant cavities electromagnetically coupled via a coupling cavity, the coupling cavity being substantially rotationally symmetric about its axis, but including an element adapted to break that symmetry, the element being rotatable within the coupling cavity, that rotation being substantially parallel to the axis of symmetry of the coupling cavity.
In such an apparatus, a resonance can be set up in the coupling cavity which is of a transverse nature to that within the accelerating cavities. It is normal to employ a TM mode of resonance with the accelerating cavities, meaning that a TE mode, such as TE111, can be set up in the coupling cavity. Because the cavity is substantially rotationally symmetric, the orientation of that field is not determined by the cavity. It is instead fixed by the rotational element. Communication between the coupling cavity and the two accelerating cavities can then be at two points within the surface of the coupling cavity, which will then xe2x80x9cseexe2x80x9d a different magnetic field depending on the orientation of the TE standing wave. Thus, the extent of coupling is varied by the simple expedient of rotating the rotational element.
This arrangement offers significant advantages over the previously described accelerators in that true variable energy output over a wider range is possible from a device which is more straightforward to manufacture and maintain. However, the resonant frequency of the coupling cell shows a small dependence on the angle of the rotateable element, as can be seen from FIG. 6. This resonant frequency is that at which the coupling cell resonates when resonances in the adjacent accelerating cells are suppressed, and is a factor affecting the degree of coupling achieved by the cell. FIG. 6 shows that as the element (according to PCT/GB99/00187) is rotated, the frequency varies sinusoidally by xc2x140 MHz. Expressed as a fraction of the mean frequency of this example, 2985 MHz, this is only a relatively small variation. However, it would be desirable to reduce or even completely remove it if possible.
One advantage of reducing or eliminating the variation of resonance frequency of this coupling cell as the element is rotated is that this would help to ensure that, at all angles of the rotatable element, an acceptable minimum separation of frequency is maintained between the resonance frequency of the desired operating xcfx80/2 mode of the coupled set of cavities and neighbouring resonance frequencies of unwanted modes of the coupled set.
The present invention therefore provides a standing wave linear accelerator, comprising a plurality of resonant cavities located along a particle beam axis, at least one pair of resonant cavities being electromagnetically coupled via a coupling cavity communicating with the resonant cavities via apertures, there being a rotationally asymmetric element within the coupling cavity adapted to rotate about a axis substantially parallel to the axis of the coupling cavity, the coupling cavity being imperfectly rotationally symmetric about its axis, the imperfection being at least due to a relative excess of material disposed within the cavity in the portion thereof opposed to the apertures.
Thus, whilst the coupling cavity is near rotationally symmetric in preferred embodiments, it departs from precise rotational symmetry by a relative excess of material which is believed to act as set out below. A relative excess of material can be provided by material. which projects inwardly into the cavity from a notional rotationally symmetric outline, or by a corresponding removal of material elsewhere.
In this respect, it is preferred that the relative excess of material comprises an inwardly directed projection on an internal wall of the cavity for ease of engineering. For maximum effect (and hence minimum extent of projection), the projection preferably extends along a length of the coupling cavity greater than the length of the apertures along the cavity axis.
Alternatively, the relative excess of material can comprise a projection extending into the cavity from an end wall thereof. For example, it can be defined by an end wall of the cavity being non-perpendicular with respect to a longitudinal axis of the coupling cavity.
In preferred embodiments of the standing wave linear accelerator, the apertures are non-identical in size. In that case, it is preferred that the relative excess of material is offset towards a location opposite the larger aperture.
It will be apparent that the present invention is a development of that shown in PCT/GB99/00187, corresponding to U.S. Pat. No. 6,376,990, an understanding of which is therefore useful in understanding the present invention. As a result, PCT/GB99/00187 (U.S. Pat. No. 6,376,990) is incorporated herein by reference and notice is given that the contents of this specification are intended to be read in conjunction with the contents of PCT/GB99/00187, and accordingly protection may be sought for the combination of features disclosed in this application and PCT/GB99/00187 (U.S. Pat. No. 6,376,990).
It is thought that this approach is effective in damping the frequency dependence of the device since as the rotatable element rotates, the E and B fields rotate accordingly. In such a coupling cavity, the E and B fields are aligned transverse to each other, and therefore the relative excess of material effectively moves from a location in a predominantly E field to a predominantly B field (or vice versa). When in a strong E field, conductive matter will tend to cause a frequency reduction. Likewise, when in a strong B field, conductive matter will tend to cause a frequency increase. Thus, as the fields rotate a variable correction is applied to the frequency. This variation is itself sinusoidally dependent on the angle of the rotatable member, but arranged to be in antiphase to the frequency dependence. Therefore the net effect can be reduced or even eliminated.
This implies that the magnitude of the relative excess of material and its location with respect to the field pattern will control the amount by which the frequency response is damped. As a result, the appropriate size of the relative excess will be dictated by its location within the E and B fields. If located in a position mid-way between the end walls of the cavity where the electric field intensity (E) and the magnetic field intensity (B) become alternately very strong as the rotateable element is rotated, the projection will have a greater effect and need not be as large as if located near the ends or edges of the cavity. It will generally be possible to arrive at suitable dimensions and locations by trial and error.